See the last 3 options - you can use a tarball file that you have prepared yourself, a tarball URL that you have hosted somewhere (like on GitHub), or a directory (folder) containing the module with package.json file.

If you are in the directory with your modified version, then you can install it globally with:

npm install -g .

Also, you can use npm link to create a globally-installed symbolic link from to the current folder, see: